So the card has a product SKU label on the backplate, possibly with an EAN number on it along with the serial number. The boxes rarely have the serial number stickers on them, but it should at least match the EAN or SKU markings near the barcode if you're concerned.
I'm not sure this card hasn't been "opened". You obviously aren't talking about the box. If you mean disassembled, then I think it
has been disassembled:
View attachment 372655
View attachment 372656
It's a little blurry but that one screw has the white sticker over it, and it looks to me like it's got a screwdriver-shaped hole puncturing it.
Ideally, a GPU of this age would have been disassembled to get re-pasted, so that punctured "warranty void if broken" sticker isn't an automatic red flag. What
is a red flag is when the seller claims "it hasn't been opened", yet there's easily visible evidence that screws on the back have been undone at least once.
It's hard to be 100% sure, the photo isn't in focus enough - but that is a cross-shaped hole in the sticker that's the exact same size as the cross slots in all the other screws!